This invention relates to electronic devices and in particular to the assembly of an electronic device on an insulative substrate.
Various processes are known in the art for assembling and packaging an electronic device, such as an integrated circuit formed on a single semiconductor chip. Such processes typically include the steps of mounting the device on an insulative substrate having a plurality of electrical conductors in a predetermined pattern thereon; electrically coupling the contact pads of the device to the substrate conductors; molding a protective plastic casing around the device; and trimming and forming electrical leads amanating from the plastic casing to form the desired package.
A common device package is the so-called "dual in line" package (DIP) wherein the electronic device is encapsulated and sealed within a molded plastic casing. The process of manufacturing DIP's is comprised of the steps of affixing the electronic device to a central mounting pad in a metal lead frame with the electrically active area of the device in an "up" or exposed position, i.e. away from the mounting pad; bonding the contact pads located on the perimeter of the device to the leads of the lead frame by soldering fine gold wires to the contact pads and leads to effect electrical interconnection between the device and lead frame; encapsulating the device and the lead frame in a plastic casing by an injection or transfer molding process; and trimming and forming the electrical leads of the lead frame to the desired length and configuration. The electrical leads emanating from the plastic casing can be bent or otherwise shaped to provide the necessary "plug in" connection to a printed circuit board or the like. Such a process is described in U.S. Pat. No. 3,882,807, assigned to the assignee of the present invention. While dual in line packaging is a popular assembly technique, it also has several disadvantages. First, the process is somewhat complicated in that it involves several different steps and requires relatively expensive bonding materials such as, for example, gold wires. Second, the contact pads of the device are located on or near the perimeter thereof and elevated above the electronic circuitry for proper electrical interconnection, thereby tending to increase the area of the semiconductor chip on which the electronic circuitry is formed.
Another technique, as described in U.S. Pat. No. 3,669,734, involves the so-called "flip chip" approach, whereby the electronic device is placed with its active surface "down" on an insulative substrate, i.e. in facing relationship with the substrate. A layer of conductive material such as copper or nickel is deposited on the contact pads of the device through openings in a passivation layer which protects the electronic circuitry. Solder material is introduced into the openings and the entire package is heated, thereby forming solder "bumps" of copper or nickel above the contact pads which are solder-bonded to selected conductors of the substrate to electrically couple the device to the substrate. Alternatively, the contact pads can be bonded to the substrate conductors by ultrasonic bonding techniques, as described in U.S. Pat. No. 3,440,027. The device is then encapsulated in a molded plastic casing and the electrical leads emanating therefrom trimmed and formed as previously described. Although the "flip chip" technique provides added protection for the electronic circuitry because the active surface is face down on the substrate, separate mounting and electrical bonding steps are still required. The electrical bonding step, in particular, is expensive because of the need for gold bonding wires, solder materials and the like.
Recently, conductive adhesives and the like have been used to replace solders and brazing alloys to bond electrical components together. Conductive adhesives typically form stronger bonds than most solders and are often less expensive than solders or brazing alloys. By adjusting the concentration of conductive metal particles in the adhesive, the conductivity thereof can be controlled for the desired application. The characteristics and applications of conductive adhesive materials are discussed in detail in a paper entitled "Conductive Adhesives, Inks and Coatings", by Justin C. Bolger et al, which was presented at the "Adhesives for Industry" Conference at El Segundo, California on June 24, 1980.